sol2.py

import math
import sys
# a = 999
# b = 1234

# 901
# 123

# a = 881
# 892

# the idea is to loop over all digits of a and for the ith digit we find a 
# number greater than or equal to a number. if at any point we find a number larger
# than the current digit, then we start checking from 0. 

# Once we generate this if it is valid we print it. If it is not valid, we do a check
# on each number within the range of digits larther than a. If one exists we print it
# otherwise we print -1

a, b = map(int, input().split())

d = {}

higherUsed = False

ln = len(str(a))

i = 0
res = ""

sa = str(a)
sb = str(b)

while i < ln:
    c = int(sa[i])

    lowerLimit = c
    if higherUsed:
        lowerLimit = 0

    nextDigit = -1
    for f in range(lowerLimit, 10):
        if f not in d:
            nextDigit = f
            break

    if nextDigit == -1:
        break

    if nextDigit > c:
        higherUsed = True

    d[nextDigit] = True

    res += str(nextDigit)
    i += 1

if i == ln and int(res) >= a and int(res) <=b:
    print(res)
    sys.exit(0)


# check for |b| > |a|
if len(sa) != len(sb):

    ourLength = len(sa) + 1
    dd = 0
    if ourLength == 2:
        dd = 10
    elif ourLength == 3:
        dd = 102
    elif ourLength == 4:
        dd = 1023
    elif ourLength == 5:
        dd = 10234
    elif ourLength == 6:
        dd = 102345

    if dd >= a and dd <= b:
        print(dd)
        sys.exit(0)



print(-1)